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IMAGING & THERAPEUTIC TECHNOLOGY |
1 From the Department of Radiology and Radiological Sciences, Vanderbilt University Medical Center, R-1308 Medical Center North, 1161 21st Ave South, Nashville, TN 37232-2675. From the AAPM/RSNA Physics Tutorial at the 1998 RSNA scientific assembly. Received July 13, 1999; revisions requested August 23 and received September 23; accepted September 24. Address reprint requests to the author.
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Abstract |
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 Top Abstract INTRODUCTIONBIOLOGIC EFFECTSOPERATIONAL HAZARDSGUIDELINES FOR MR IMAGING...CONCLUSIONSReferences |
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Index Terms: Magnetic resonance (MR), biological effects • Magnetic resonance (MR), safety
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INTRODUCTION |
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TopAbstractINTRODUCTIONBIOLOGIC EFFECTSOPERATIONAL HAZARDSGUIDELINES FOR MR IMAGING...CONCLUSIONSReferences |
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Concerns about static magnetic fields now relate primarily to risks associated with metallic objects becoming projectiles in the influence of the static field and with interactions with implants and other electrical devices. In addition, burns resulting from radio-frequency (RF)–induced currents in conducting loops, either electrical leads or from inadvertent body geometries, continue to be reported. With the introduction of high-performance gradients for echo-planar imaging, the potential now exists for peripheral nerve stimulation from rapid gradient switching.
The purpose of this article is to review the current level of knowledge about the safety of MR imaging, including the biologic effects from static magnetic fields, RF fields, time-varying gradient fields, and acoustic noise; the hazards associated with each of these; and the safety recommendations. Guidelines for maintaining a safe MR imaging environment and screening patients are also discussed, with references to safety resources, relevant research, and current FDA guidelines and recommendations.
Current guidelines on RF specific absorption rates (SAR), the rate of change of gradient fields with time (dB/dt), and acoustic noise levels can all be found at the FDA web site (www.fda .gov). The reader is also referred to the book by Shellock and Kanal for a general review of all aspects of MR imaging safety (1). Other general reviews are also available in the literature (2,3). A wide spectrum of questions and answers on MR imaging safety can be found on the Shellock and Kanal web site (http://Kanal.arad.upmc .edu/mrsafety.html).
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BIOLOGIC EFFECTS |
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TopAbstractINTRODUCTIONBIOLOGIC EFFECTSOPERATIONAL HAZARDSGUIDELINES FOR MR IMAGING...CONCLUSIONSReferences |
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A known reversible biologic effect associated with exposure to a strong static magnetic field is the elevation of the T wave in electrocardiographic tracings. Because blood is a conductive medium, the movement of blood in the magnetic field of the MR imaging unit causes a magnetohydrodynamic effect that produces a voltage across the vessel. The mechanism responsible for the induced voltage depends on the velocity of the blood flow, the strength of the external magnetic field, the diameter of the vessel, and the angle of the blood flow with respect to the direction of the magnetic field (Fig 1). Because the peak flow rate occurs during the T-wave interval of the electrocardiographic tracing, the added voltage from the flowing blood manifests as an elevation of the T wave (Fig 2).
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RF Fields
No convincing evidence exists for direct nonthermal biologic effects of RF radiation resulting from diagnostic MR imaging (7–9). There is, however, clear evidence that significant RF burns can and do occur (8–12). Precautions to avoid RF burns in patients are an essential component of MR imaging safety and are discussed in a later section.
Time-varying Gradient Magnetic Fields
Time-varying gradient magnetic fields have recently been reconsidered for biologic effects with the introduction of rapid echo-planar imaging and the use of high-performance gradient systems, since it is known that rapidly switching magnetic fields can stimulate muscle and nerve tissue. The specific concern has been peripheral nerve stimulation (13,14). In response, the FDA has established guidelines that limit switching rates at a factor of three below the mean threshold for peripheral nerve stimulation. Incidentally, this level is a factor of 30 below the threshold for cardiac stimulation.
At present, there is no known mechanism that would suggest an irreversible biologic effect caused by rapidly switching magnetic fields.
Acoustic Noise
Guidelines for acceptable acoustic noise levels associated with MR imaging are based on Occupational Safety and Health Administration guidelines for industrial workers and are set at maintaining root mean square (rms) levels below 100 dB with hearing protection in place. With certain pulse sequences, it is possible to exceed these guidelines (15,16). It is always prudent to ensure that patients are supplied with hearing protection regardless of what particular pulse sequence is used.
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OPERATIONAL HAZARDS |
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It is generally accepted that patients should be educated about the risks associated with the MR imaging procedure during the screening interview (17,18). However, it is sometimes overlooked that it is equally necessary to educate all facility employees who may have even a remote chance of entering the high-magnetic-field area of the MR imaging unit. It is obvious that all medical personnel (technologists, physicians, nurses, escorts, etc) must be trained, but it may not be obvious that maintenance, security, and custodial personnel who may be present after normal working hours must also be educated about risks associated with the MR imaging facility. A complete and coordinated educational program must therefore be a part of a safe MR imaging facility.
The educational program should include information about the behavior of metallic objects in static magnetic fields and the hazards associated with those fields, the hazards accompanying RF fields, and those associated with time-varying magnetic fields, with substantial emphasis on patient screening and preparation to avoid accidents and RF burns.
Review of the Effects of Magnetic Fields on Ferromagnetic Objects
The field from a conventional MR imaging unit can be characterized in terms of two spatial regions. Region 1 is the area surrounding the isocenter of the magnetic field that is contained within the bore of the magnet. The magnetic field in this region is relatively constant and uniform in strength. Region 2 is the field external to the physical magnet and is a gradient field; that is, the strength of the field varies with spatial position. Metallic objects within region 1 experience only a rotational force called a torque. In region 1, a nonspherical metallic object either inside or outside the body will experience a torque, depending on the size and shape of the object. The induced rotational motion of an object inside the body can cause the object to tear surrounding tissue.
The field strength in region 2 decreases with distance from the magnet. A metallic object located in the gradient field of region 2 may experience both rotational and translational forces. The direction of the translational force will be in the direction of the higher field strength (toward the center of the magnet) (Fig 3). In region 2, a metallic object within the body will experience a translational force, the strength of which depends on the size of the object and the strength of the gradient field. (The magnitude of the gradient field is usually greatest in the regions immediately adjacent to the magnet). Objects within the body will be caused to accelerate toward the magnetic isocenter and can thus tear the surrounding tissue. The effects of the field in region 2 will be experienced by the patient as he or she approaches the magnet before actually being placed within the magnet bore.
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Hazards Associated with Static Magnetic Fields
Implanted or Foreign Metallic Objects.—One of the most substantial hazards encountered by those working around strong magnets is the attraction of the magnet to ferromagnetic objects. It is a common misconception that all stainless steel is nonferromagnetic. This is definitely not true, and clips or other implants of stainless steel construction should not be assumed to be nonmagnetic. Any ferromagnetic object close to the magnet—even objects in the soft tissues of a patient who enters the magnetic field of an MR imaging unit—will experience a magnetic pull and, depending on the shape and orientation of the object, a rotating torque. The object can thus be dislodged from its site and moved, resulting in cuts and tears in the surrounding tissue.
Two specific metallic objects that may be encountered in patients and that can be adversely affected by magnetic fields have been identified: (a) metallic fragments in the eye and (b) intracerebral aneurysm clips. It is unfortunate that these objects were identified after disastrous accidents, one of which resulted in the death of a patient. Magnetic movement of metallic fragments in the eye has led to blindness (19,20). Movement of a metallic aneurysm clip in the brain has led to at least one fatality when the clip tore the middle cerebral artery (21). Older aneurysm clips were made with a magnetic steel spring. Newer clips have been tested and declared safe for MR imaging (22,23). A patient presenting for an MR imaging examination and having an aneurysm clip should not be imaged unless definitive proof supporting the safety of the clip can be produced.
Certain intravascular implants are known to become firmly incorporated into the vessel wall about 6 weeks after introduction. It is therefore considered safe to image patients with most intravascular coils, filters, and stents after a suitable time has elapsed (1). As always, it is essential to carefully document the characteristics of any implanted device before imaging is attempted and to specifically refrain from imaging if there is any reason to believe that the device is not held firmly in place.
In general, the presence of any electrically active implant should be considered a contraindication for MR imaging until proof of its safety is demonstrated. Literature references are available specifically for cochlear implants (24), implanted magnetic eye sockets (25,26), neurostimulators (27), and cardiac pacemakers (2,28–30). At least one new neurostimulator is now being evaluated for its MR imaging safety. The FDA maintains a database on adverse incidents that have occurred with medical devices. The Medical Device Report database is available for review on the FDA web site (www.fda.gov) and contains the complete description of specific events. The database specifically contains accounts of instances in which individuals with metallic objects in their bodies were injured when they came into the gradient fringe field of the MR imaging unit.
Metallic Projectiles.—One of the most serious risks associated with MR imaging is the hazard of ferromagnetic objects inadvertently being brought into the influence of the strong magnetic field of the MR imaging unit. Depending on the size and closeness of the object to the magnet, the pull on the object can often exceed the strength of an individual to restrain the movement of the object. There have been documented cases of oxygen cylinders, intravenous fluid poles, so-called sand bags (which actually contain iron shot), mop buckets, vacuum cleaners, and numerous medical devices becoming projectiles and being accelerated into the magnet.
Anyone, staff or patient, in the path between the metallic object and the center of the magnet could be seriously injured or killed. Numerous incidents of ferromagnetic projectiles causing injury have been reported (17, www.fda.gov). Because accidents can happen at anytime, awaiting only a lapse in security, identifying all metallic objects is a high-priority component of patient and staff screening.
The "5-Gauss Line."—The most commonly recognized safety policy is the so-called 5-G (0.0005-T) line. The criterion of a 5-G (0.0005-T) line was established very early to limit access of individuals with cardiac pacemakers into high-magnetic-field areas that could potentially disable the pacemaker. The safety policy requires that physical barriers and appropriate signage be employed to exclude access by members of the general public to areas in which the static magnetic field strength is greater than 5 G (0.0005 T). Additional information is available at www.fda.gov.
It is important to stress that the magnetic fringe fields extend in three dimensions and that the 5-G (0.0005-T) field restrictions must be applied in both the horizontal and vertical planes. Specifically, one must consider the fringe field extent into the floors above and below the magnet. The extent of the 5-G (0.0005-T) line depends on the location of the magnet isocenter and the strength of the magnet. The locations of the specific fringe field line should be supplied to the MR imaging site by the magnet vendor.
The signage used to indicate the 5-G (0.0005-T) line should not only clearly identify the existence of the high-magnetic-field area but also describe potential consequences. These signs may contain comments that point out the potentially life-threatening circumstances that could result from bringing ferromagnetic objects into the high-magnetic-field region (ie, the projectile effect). The signs should also include warnings about the potential for disabling cardiac pacemakers and electronic implants and about the damages that could occur to magnetic data storage devices (computer disks and credit cards), cameras, watches, and other electrical and electronic devices.
Operational Safety Rules.—MR imaging facilities should consider implementing the following specific operational rules to maintain security and safety in high-magnetic-field areas:
1. Access to the high-magnetic-field area should be limited to trained personnel or to screened patients and visitors who are accompanied by trained personnel.
2. Entrance to the high-magnetic-field area should be controlled by a lockable door, and the keys to the area should be issued only to trained personnel.
3. All entrances to the high-magnetic-field area should be visible to the system operator.
4. All visitors must be screened by the operator before entry is allowed.
5. Appropriate warning signs must be posted.
Hazards Associated with RF Fields
A substantial hazard of MR imaging that may often be overlooked is that of the associated RF effects. The scientific evidence for biologic effects of RF suggests that direct tissue heating leading to burns is the most important hazard of RF fields. Currently, RF burns that result from the inductive heating of conductors and leads for patient monitoring constitute the majority of MR imaging–related injury reports (8–12,31–33).
Direct tissue heating by RF is a function of many parameters, including the mass, conductivity, and heat capacity of the body part under study; its level of blood perfusion; ambient temperature; air circulation; the specific pulse sequence; and RF frequency. The FDA has used the tissue RF SAR as the parameter of heating to establish guidelines for allowable RF energy deposition.
Specific Absorption Rate.—The SAR is expressed in units of watts per kilogram of body weight (W/kg). The general guideline used by the FDA to establish allowable RF energy deposition is based on levels that produce a maximum change in tissue temperature of 1° C. According to the specific FDA criteria for SAR limits, the SAR must be no greater than (a) 4 W/kg averaged over whole body for any 15-minute period, (b) 3 W/kg averaged over the head for any 10-minute period, or (c) 8 W/kg in any gram of tissue in the head or torso or 12 W/kg in any gram of tissue in the extremities for any period of 5 minutes.
RF Burns.—Typical RF-related injuries recorded in the Medical Device Report database include burns from a conductive lead placed against the patient's bare skin during an MR imaging procedure, blisters on a patient's finger from a non–MR imaging compatible pulse oximeter, and a burn to the hand of an anesthetized child from an electrocardiographic cable. Care must be taken to ensure that all conductive leads are placed such that they do not form loops and that they are insulated from contact with bare skin through the use of sheets or other thermal and electrical insulating materials.
Loops of conductors within the magnet bore can present substantial risk for RF burns as a result of inductive heating of the conductor from the time-varying RF field. The rapidly changing magnetic field will induce an electromotive force or voltage in the conductor that causes a flow of current. The flowing current in a conductor with electrical resistance will result in heating the conductor; thus, the conductor in contact with the skin can cause a burn. The maximum induction occurs when the plane of the conducting loop is perpendicular to that of the changing magnetic field (Fig 4). Inductive heating can also occur with receive-only surface coils when the active-decoupling circuitry fails. For this reason, surface coils should also be insulated from the patient's skin.
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Hazards Associated with Time-varying Gradient Magnetic Fields
Rapidly switching magnetic fields can stimulate muscle and nerve tissues. The mean threshold levels (measured in tesla per second) for various stimulations are 3,600 T/sec for the heart, 900 T/sec for the respiratory system, 90 T/sec for pain, and 60 T/sec for the peripheral nerves. It should be stressed that these are mean threshold levels; that is, the stimulation thresholds in a single individual may be either higher or lower than these values.
The general FDA guidelines for switching magnetic fields have been established to limit gradient-switching rates to a factor of three below the mean peripheral nerve stimulation level. Gradient waveforms may have either a sinusoidal or square wave switching pattern. A switching time (
) is defined for a square wave pattern as the time needed to go from zero gradient to the maximum gradient strength. For sinusoidal switching, is generally understood to represent the time for one-half of a total switching cycle.
Stimulation threshold values vary not only from person to person but also depending on gradient direction. Gradients along the long axis of the body (z) have a lower stimulation threshold than do gradients oriented transverse (x, y) or perpendicular to the long axis of the body (13,14). The FDA guidelines take gradient orientation into account and have specified limits that allow a factor of three higher levels for transverse gradients relative to axial gradients.
The FDA guidelines are also specified as a function of the switching rate (). For an axial gradient (z) specifically,
A graphical illustration of the stimulation thresholds and FDA guidelines is shown in Figure 6.
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GUIDELINES FOR MR IMAGING FACILITY MANAGEMENT |
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TopAbstractINTRODUCTIONBIOLOGIC EFFECTSOPERATIONAL HAZARDSGUIDELINES FOR MR IMAGING...CONCLUSIONSReferences |
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It is the responsibility of the specific health care provider interviewing the patient to determine if the requested MR imaging procedure would be safe for that particular patient. If a patient has been occupationally exposed to metal fragments, the screening should include either radiography or computed tomography of the orbits to rule out the presence of metal fragments in the eyes (17). If there is any question, the MR imaging procedure should be postponed pending consultation with the radiologist in charge or until additional information is gathered. In some cases, the decision must be to cancel the study because of a contraindicating condition. In general, each of the following conditions must be considered as an absolute contraindication to MR imaging until information to the contrary is obtained: (a) the presence of an active electronic device (cardiac pacemaker, cochlear implant, nerve or bone stimulator) in the body, (b) the presence of a cerebral aneurysm clip, (c) the presence of intraocular metal fragments, (d) the presence of ferromagnetic foreign bodies, (e) magnetic eye sockets, or (f) the presence of any unfamiliar device.
Patient safety is of the utmost concern, and thus one must be very conservative about any safety decision. On the other hand, a thorough knowledge of MR imaging safety and a familiarity with the literature may prevent the unwarranted cancellation of a needed study. In that regard, not all metallic implants should be considered an absolute contraindication to MR imaging. In general, internal orthopedic hardware; extracranial surgical clips; staples and wires in the body (specifically excluding cerebral aneurysm clips); intravascular stents, coils, and filters; and essentially all dental devices are generally safe and pose no substantial hazard to the patient. However, even though these objects are safe, they may produce susceptibility artifacts that render the images near the object nondiagnostic. All of these devices, although generally safe, must be confirmed as verified safe in each individual circumstance by using medical documentation from either the device vendor or references such as Shellock and Kanal (1).
Once the patient has completed the screening questionnaire, all responses must be read carefully and additional questions should be asked if some responses raise concern. After an acceptable screening interview, the patient should then be prepared to enter the MR imaging examination room. The most desirable patient preparation from a purely safety perspective is to have the patient remove his or her shoes, outer clothing, all metallic items (hair clips, pins, belts, and jewelry), and any underclothing with metal or wire and to don a hospital gown or surgical scrubs. If the use of gowns or scrubs is not possible, the patient must be carefully screened for any metal before entering the examination room. Any metallic items in question should be checked with a small magnet before the patient enters the room to ensure that no ferromagnetic items are taken into the room.
Pregnant Patients
The safety of MR imaging procedures for pregnant patients was one of the first concerns raised when MR imaging became an approved medical imaging procedure. In 1997, the American College of Radiology issued a statement on the safety of MR imaging in pregnant patients. The full text of this statement can be found in the ACR's 1997 Radiology Practice Standards. The essence of the statement is that in light of the lack of data demonstrating deleterious effects of MR on the developing human fetus, MR imaging should be recommended for evaluating pregnant patients when any alternative imaging procedure involves ionizing radiation. The statement further indicates that each case should be considered on an individual basis and that MR imaging should be performed only after the approval by the attending radiologist.
Pregnant Workers
A different but related question is the safety of pregnant MR imaging workers. To evaluate the potential hazard that might be posed to the pregnant worker from the MR imaging environment (static magnetic fields, switched gradient fields, and RF fields), Kanal, Shellock and Savitz (34) conducted an epidemiologic study of MR imaging technologists in the United States. They concluded that their data were negative with respect to any statistically significant elevations in the rates of spontaneous abortion, infertility, and premature delivery (34).
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CONCLUSIONS |
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TopAbstractINTRODUCTIONBIOLOGIC EFFECTSOPERATIONAL HAZARDSGUIDELINES FOR MR IMAGING...CONCLUSIONSReferences |
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A quotation from the FDA draft document published February 7, 1997, can serve as a summarizing statement: "For a properly operating system, the hazards associated with direct interactions of these fields [static magnetic, pulsed gradient, and RF] and the body are negligible. It is the interactions of these fields with medical devices...that create concerns for safety."
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Footnotes |
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LEARNING OBJECTIVES After reading this article and taking the test, the reader will be able to: • List current FDA guidelines and recommendations about MR imaging safety. • Make recommendations for patient screening before MR imaging procedures. • Make recommendations for maintaining a safe environment for both patients and workers.
Abbreviations: FDA = Food and Drug Administration RF = radio frequency SAR = specific absorption rate
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References |
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TopAbstractINTRODUCTIONBIOLOGIC EFFECTSOPERATIONAL HAZARDSGUIDELINES FOR MR IMAGING...CONCLUSIONSReferences |
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) with respect to the magnetic field (B) will produce a force (F) on the charge carriers in the blood. This force in turn produces a voltage difference across the vessel in a direction perpendicular to the blood flow. The magnitude of this electromotive force (emf) is equal to the product of the vessel diameter (d), the velocity (V), the field strength (B), and the sin of the angle between the direction of the blood flow and the magnetic field.
) in the presence of a conductive loop with area A. The time-varying magnetic field produced by the RF coils will induce an electromotive force (emf) in the coil, which will in turn cause a current flow. The magnitude of the force is given by: emf = 
